1. Field of the Invention
The present invention relates to a small dual frequency band, dual-mode, feedhorn and, more particularly, to a small feedhorn comprising in sequence a first waveguide section capable of supporting a dominant TE.sub.11 mode in two separate frequency bands, a first discontinuity for converting a portion of the TE.sub.11 mode in each frequency band into the TM.sub.11 mode, a second waveguide section for propagating the TE.sub.11 mode in both frequency bands and only the TM.sub.11 mode in the higher frequency band, a second discontinuity for converting a portion of the TE.sub.11 mode in both frequency bands into the TM.sub.11 mode, and a third waveguide section comprising a length such that the TE.sub.11 mode energy and the vector sum of associated TM.sub.11 mode energies in both frequency bands are in phase at the exit port of the feedhorn.
2. Description of the Prior Art
Horn antennas as well as devices and techniques for exciting higher order modes in horns for improved performance are well known in the art. One such technique is to introduce one or more abrupt symmetrical steps within the guide as shown, for example, in U.S. Pat. Nos. 3,305,870 issued to J. E. Webb on Feb. 21, 1967; 3,510,875 issued to D. E. Beguin on May 5, 1970; and 4,122,446 issued to L. H. Hansen et al on Oct. 24, 1978. Alternative techniques to the abrupt step is the use of a groove or iris within the guide as shown, for example, in the article "A New Horn Antenna With Suppressed Sidelobes and Equal Beamwidth" by P. D. Potter in The Microwave Journal, Vol. VI, No. 6, June 1963 and "Mode Conversion in Circular Waveguides" by E. R. Nagelberg et al in BSTJ, Vol. XLIV, No. 7, September 1965 at pages 1321-1338.
Still another technique for mode conversion is to use a circular dielectric rod having tapered ends mounted coaxially within a conical horn as shown, for example, in U.S. Pat. No. 3,605,101 issued to N. J. Kolettis et al on Sept. 14, 1971. An alternative configuration using dielectrics for mode conversion uses dual dielectric bands mounted within a flared guide as shown in U.S. Pat. No. 4,141,015 issued to M. N. Wong et al on Feb. 20, 1979 for improving the rotational symmetry or ellipticity of the radiated beam. Still another dielectrically loaded horn antenna for dual-band use is shown in the article "Dielectric-Loaded Horn Antenna" by T. Sato in Electronics and Communications in Japan, Vol. 54-B, No. 9, September 1971 at pages 57-63 where dielectric strips are mounted within the horn. However, such dielectric belts or strips must be accurately positioned on the tapered section of the horn to obtain the proper effect, and such placement is critical.
The problem remaining in the prior art is to provide a dual frequency band, dual-mode horn which is easily achieved by the use of less critical techniques than found with the prior art arrangements and which will operate over two narrow bands separated by about 20 percent.